The present invention relates generally to digital scanners, and more particularly to an arrangement and method for detecting and correcting defects from scanner calibration references.
One of the present uses of scanners is in the printing industry, and particularly in a production-printing environment. A digital scanner is a device that reads an image which is placed on the platen of the scanner and generates a digital image of the document. The scanner uses a light source to illuminate the document. Light is reflected from the light and dark areas of the image on the document, in proportion to the lightness of the original document. The light is passed by a focusing system, such as mirrors and lenses, to photodetector elements. The image is sequentially resolved into small points which are commonly referred to as pixels by the photodetector elements. The photodetector elements, in turn, convert the light into a proportional electrical signal at each point. In this manner, the tones of the original document are resolved into a pattern of proportional voltages. The photodistributions are arranged in lines and rows to form an array. After the individual voltages have been converted into an electrical signal, electronic circuitry may then be used to manipulate the image, such as changing the reproduction ratio or reversing black and white.
In a production printing environment, after the signal processing has been performed by the electronic circuitry, the resulting electrical signal is transmitted to a printer which uses the electrical signal to generate a document by modulating a light beam that scan across a photosensitive drum.
The photocells in the scanner may be any suitable photoelectric converter such as photodiodes, phototransistors and Charge-Coupled Devices (CCDs). CCDs are made up of a row of several thousand small photocells, each only a few microns square. The associated circuitry which is required for the scanning function is also built into the CCDs so the complete photoelectric converter is a one-chip microcircuit. The resolution of the original image will be proportional to the size of each pixel, so the image is resolved into blocks a few microns square. Generally, the level of CCD output signals decreases with an increased amount of light falling on each pixel. The electrical signals received from the CCD represent, in analog form, the (halftone) image density recorded by each pixel. The analog signals are converted to an equivalent digital signal by an analog to digital (A/D) converter. The digital signal from each of the lines are read and stored sequentially into a line memory.
Typically, a scanner will include shading-correction circuitry or may run shading-correction algorithms on a processor. Shading correction is necessary because the output of all the pixels of the CCD will not always be uniform even if the original is uniform in density across a fill scan line. There are various reasons for the non-uniformity. For example, the intensity of the scanning lamp may vary between its center and ends. Illumination lamps may have, for example, eight filaments, which may be a cause of non-uniformity. The amount of light that passes through the lenses may also be different for the center and edge of the lenses also causing non-uniformity. Additionally, the sensitivity of the several thousand individual photocells in the CCD may vary from the others. Further, as described below, commonly the sensor architecture may have several video channels. Differences in the channels will occur because the electronic gain and offset channels may not be matched perfectly. Additionally, the channel structure is arranged such that a seam or butt exists in the middle of the device between the channels. Accordingly, there is a need to match the various channels of data at the center of the device so that the seam or butt is not observable.
Due to the inherent nature of the non-uniformity of the signals reproduced by the CCD, it is desirable to provide efficient and reliable calibration arrangements and methods to compensate for the non-uniformity.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. The present invention is directed to a method for calibrating a scanner. First, the scanner to be calibrated scans a light reference strip. An algorithm is used to determine if a defect exists in the light reference strip. Data is excluded if the data is data from a defect in the light reference strip. The scanner is then calibrated with the data from the scanning of the light reference strip excluding data contributed by the defect in the light reference strip.